Footwear generally comprises a bottom unit or sole and an upper. The sole may be a single layer or piece, or may comprise two or more layers or pieces. Three layer soles are common, comprising a bottom or outsole, an insole or foot bed, and a midsole therebetween. The sole has a heel portion, forefoot portion and midfoot portion. The upper may be leather or fabric continually adhered to the periphery of the sole, as is the case with shoes and boots. In the case of sandals, one or more straps are attached to the sole at selected points, which straps are adapted to encircle the user's foot.
The weight bearing surfaces of the sole are the heel and forefoot portions. For cushion and comfort, it is desirable to have relatively soft forefoot and heel portions. The midfoot portion of the sole should support the arch of the user's foot, but generally does not touch the ground for both functional and aesthetic reasons. The midfoot portion of the sole thereby acts as a bridge between the forefoot and heel portions of the sole. Accordingly, it is desirable to stiffen the midfoot portion of the sole in order to prevent undesirable reverse flex and promote correct support of the arch. A midfoot stiffening means is a shank.
Shoe soles tend to twist due to natural weight shifting during walking or running. When one walks, the heel is the first portion of the foot to contact the ground. As the step continues and the remainder of the foot contacts the ground, the weight of the body is carried forward along the lateral side of the foot. As the heel leaves the ground, the weight of the body shifts back towards the medial side of the foot. The medial forefoot region near the ball of the foot and the big toe is the last portion of the foot to leave the ground. This weight shifting to the lateral and back to the medial side of the foot during the natural gait exerts torsional forces on the sole and may result in undesirable twisting of the shoe sole. In addition to supporting the user's arch, a midfoot shank also provides torsional rigidity.
The need for a resilient and stiff sole is critical in the case of sandals. For this type of footwear, attachment of the sole to the foot occurs via straps. As these straps are connected to the sole at only a few points, the sandal sole will receive less restraint or control from the top of the foot than will the sole of a shoe or boot which is connected around the entire periphery of the sole. The sandal sole, therefore, should have superior structural integrity to retain a useful shape during use. Without a stiff sole, sandals tend to undesirably "flop" away from the foot and to twist during use. As sandal soles tend to "fold" and twist, a relatively stiff and resilient shank is desirable to insure longitudinal and torsional rigidity, and to allow for a natural stride. The need for a stiff shank is thus particularly acute in the case of sandals, but not limited to sandals.
In the prior art several techniques have been developed to provide a desirably stiff and resilient shanks for shoes and sandals. One type of shank is a metal plate that is inserted either between the midsole and outsole, or between the midsole and insole. The plate bridges the gap between the heel and forefoot regions of the sole. A second type of shank is formed by gluing or molding a plastic component to the midsole or outsole that acts to stiffen the midfoot area. A third type, known as a "fiddle shank," is a laterally narrow section of rubber or other material, commonly molded to the bottom of the outsole. The fiddle shank extends downwardly from the outsole to selectively touch the ground and thereby support the arch region of the foot.
Durable, permanent attachment of the shank to the sole can be a problem, however. Attachment can be achieved through use of an adhesive, by mechanical attachment means, by a secondary molding step, or by insertion of a shank into a pocket within the sole. These means of attachment commonly are disadvantageously subject to delamination or separation due to excessive wear or defective attachment. Also, such means of attachment typically allow some movement of the sole relative to the shank, thereby diminishing desirable stiffening effects of the shank. Also, such means of attachment often requires additional time consuming and costly manufacturing step (s).
Another drawback of prior art shanks is that the shank is often a relatively thin, flat member. The thin, flat configuration is a poor shape to resist torsional and bending forces. It also does not fully support the arch area of the foot because it does not directly contact the arch area.
An unresolved need therefor exists for an improved stiff shank structure for footwear for arch support and torsional rigidity, with a relatively soft forefoot and heel portions of the sole, while maintaining cost effective manufacturing techniques, and which provide durable solutions.